Phenolic Profile and Antioxidant Capacity of Walnut Extract as Influenced by the Extraction Method and Solvent

Total phenolic content, total flavonoid content, antioxidant activity and individual phenolic compounds were assessed in full fat and defatted walnut kernel. For quantification of phenolic fraction of walnut kernels, two different solvents (methanol and ethanol) and two methods of extraction (ultrasonic-assisted extraction and Soxhlet extraction) were tested. Total phenolics, flavonoid content and antioxidant capacity of alcoholic extracts varied depending on the solvent used and extraction methods. Seventeen phenolic compounds were detected and the study provides evidence on high phenolic contents and high antioxidant potential of full fat walnut kernel and defatted walnut kernel. The Soxhlet extraction is the best in terms of the amounts of total phenolic content (2,089.2 mg gallic acid equivalent/ 100 g dry matter), while the ultrasonic assisted extraction is a fast method but resulted in significantly lower phenolic content (667.3-1,426.8 mg gallic acid equivalent / 100 g dry matter). The concentrations of phenolics (especially (+)-catechin hydrate, juglone, vanillic acid, caffeic acid, ferulic acid, sinapic acid, salicylic acid and ellagic acid) are many fold lower in ultrasonic-assisted extraction as compare to the Soxhlet method using the same extraction solvent. The results of this study provide evidence on high phenolic contents and high antioxidant potential of full fat and defatted walnut kernel.